I'm sure this isn't new to anyone, but I'm using this safe breeder using thorium cells:
Safe high speed thorium breeder
It re-enriches 4 cells each 459 seconds (115 sec each), it's also easy to automate using buildcraft pipes.
It's also made to be expandable, you can start without chambers and 15 heating cells and as you get copper you expand, and slowly increase the number of heating cells.